Intervertebral implants are known, for example, from U.S. Pat. No. 5,258,031. The displacement of the highest point and of the lowest point of the crowned joint surfaces, i.e., of the plane in which the center of the crowned joint surfaces is located (in the direction of the rear edge of the implant) corresponds to the anatomic conditions. Adjacent vertebral bodies pivot during their mutual pivoting movement around a fulcrum that is located between the middle and the dorsal end of the intervertebral disk, and the ball and socket joint is correspondingly positioned in the dorsal third of the intervertebral disk in the prior-art prosthesis. However, the consequence of this in the prior-art arrangement is that the supporting forces are transmitted to the carrier plates via three relatively small crowned joint surfaces exclusively in the rear third. These carrier plates consequently cannot transmit the supporting forces uniformly to the vertebral bodies, but the supporting forces will occur essentially in the rear third. This may lead to undesired break-ins of the carrier plates in the dorsal area of the vertebral body, especially in the case of weakened vertebral bodies.
Accordingly, there remains a need to improve an intervertebral implant of this class such that uniform introduction of the forces via the vertebral body is also achieved when the fulcrum of the intervertebral implant is displaced in the dorsal direction.
Numerous embodiment variants of such artificial intervertebral disk prosthesis are already known. Many of the prior-art solutions are based on the technical principle of the ball and socket joint.
Thus, WO 01/01893 (Prodisc) describes an intervertebral disk prosthesis with two metallic end plates and an intermediate part made of polyethylene with a convex bearing surface, which slides against a concave surface in one of the two metal plates. The rotation center is located centrally in this prosthesis in the middle between the front and rear edges of the metal plates. U.S. Pat. No. 5,258,031 describes, in contrast, an intervertebral disk prosthesis with a dorsally located rotation center.
The object of the present invention is to optimize the existing designs for intervertebral disk prostheses based on the ball and socket joint principle in terms of wear, kinematics and load distribution.